In unstimulated, isolated, rat ventricular muscle, the increase in resting force (DeltaRF) which occurs with an increase in external calcium concentration (Delta[Ca++]e) can be largely abolished by lowering cellular sodium content by removing sodium from the superfusate and can be potentiated by raising cellular sodium content by adding ouabain. We hypothesized that this demonstrated the activity of a membrane Na/Ca exchange (see ZO1 AG 00221-01 CPB). Since this membrane exchange is well known to exhibit graded activity, we sought to extend our observations by demonstrating a graded response between [Na+]i and DeltaRF. External potassium concentration ([K+]e) was set at gradually lowered values for each experiment in the series to give graded inhibition of the Na-K pump and thus graded elevation of [Na+]i. [Ca++]e was then increased from 0 to 2 mM and the DeltaRF recorded. The results show that increasing DeltaRF occurs with decreasing [K+]e (and thus increasing [Na+]i. Thus, not only does the DeltaRF for a Delta[Ca++]e depend on [Na+]i, but does so in a graded manner. This is furher evidence for the activity of the Na/Ca exchange in controlling DeltaRF in isolated rat muscle.